FIG. 1 illustrates an exemplary miniature projector display device 100, sometimes referred to as a picoprojector. The miniature projector device 100 can be integrated with or attached to a portable device, such as, but not limited to, a mobile phone, a smart phone, a portable computer (e.g., a laptop or netbook), a personal data assistant (PDA), or a portable media player (e.g., DVD player). The miniature projector device 100 can alternatively be integrated with or attached to a non-portable device, such as a desktop computer or a media player (e.g., a DVD player), but not limited thereto. The miniature projector device 100 can alternatively be a stand alone device.
Referring to FIG. 1, the projector display device 100 is shown as including a video source 102, a controller 104 (e.g., an application specific integrated circuit and/or a micro-controller), a laser diode driver (LDD) 108 and a voltage regulator 110. Depending on the type of video source, a video analog-font-end (AFE) can be included between the video source and controller, and the video AFE may include, e.g., one or more analog-to-digital converters (ADCs). For example, if the input is a Video Graphics Array (VGA) input, then a video AFE may be included. However, a video AFE may not be needed where the video source is a digital video source.
The controller 104 can perform scaling and/or pre-distortion of video signals before such signals are provided to the LDD 108. The voltage regulator 110 (e.g., a quad-output adjustable DC-DC buck-boost regulator) can convert a voltage provided by a voltage source (e.g., a battery or AC supply) into the various voltage levels (e.g., four voltage levels V1, V2, V3 and V4) for powering the various components of the projector display device 100.
The LDD 108 is shown as including three digital-to-analog converts DACs 1091, 1092 and 1093 (which can be collectively referred to as DACs 109). The LDD is also shown as including a serial interface 122 which may receive, via a serial bus 103, a serial enable (SEN) signal and a serial clock signal (SClk) from a serial interface of the controller 104. Additionally, a bi-directional serial data input/output (SDIO) line of the serial bus 103 allows the controller 104 to write data to and read data from registers within the LDD 108. Alternative serial buses and interfaces can be used, such as, but not limited to, an Inter-Integrated Circuit (I2C) or an Serial Peripheral Interface (SPI) bus and interface. The LDD 108 also includes registers, and the like, which are not shown.
The DACs 109 of the LDD 108 drive laser diodes 112, which can include, e.g., a red, a green and a blue laser diode, but are not limited thereto. Where the LDD 108 is used to drive a red (R), a green (G) and a blue (B) laser diode, the LDD can be referred to as a RGB triple laser diode driver.
The light produced by the laser diodes 112 can be provided to beam splitters 114, which can direct a small percentage of the light toward one or more calibration photo-detectors (PDs) 120, and direct the remainder of the light toward projector optics 116, which include lenses, mirrors, reflection plates and/or the like. The light output by the optics 116 can be provided to one or more micro mirror(s) 118. The mirror(s) 118 can be controlled by the controller 104, or another portion of the system, to raster-scan reflected light onto a surface, e.g., a screen, a wall, the back of a chair, etc. Because of the scanning of laser beams performed using the mirror(s) 118, the projector 100 can be referred to as a laser based scanning projector 100. In one configuration, a single mirror 118 that can be controlled in both the X and Y directions is used for scanning of the laser beams. In another configuration, a first mirror 118 is used for controlling horizontal scanning (i.e., scanning in the X direction), and a second mirror 118 is used for controlling vertical scanning (i.e., scanning in the Y direction). These are just two exemplary configurations, which are not meant to be limiting. It is also possible that more than two mirrors 118 be used.
In a laser based scanning projector, at each clock cycle, the R, G, and B lasers diodes output a pixel intensity at a location set by the linear speed of the scanning mirror(s) 118 and a clock time base, as can be appreciated from the exemplary timing diagram of FIG. 2A. In the exemplary timing diagram of FIG. 2A, there are only 8 pixels per horizontal line, and there is no output during each blanking period (B). However, it is noted that there are typically many more pixels per line in a normal display. At each clock cycle, each color data pixel intensity can be either controlled using a pulse width modulation (PWM) scheme, where the R, G and B lasers diodes are turned on for different durations, or by amplitude modulation (AM), where the R, G and B laser diodes may all be driven at the same time but with different current levels.
Over time, a laser beam pointing direction might shift for various reasons, which causes misalignment among pixel colors. This is illustrated in timing diagram of FIG. 2B, which represents the observed timing relative to the image being displayed (as opposed to the timing of the data being sent from the controller 104 to the LDD 108). FIG. 2B attempts to illustrate that the blue laser shifts to the right hand side (RHS) by one pixel (or close to one pixel), resulting in a color offset in the displayed image, which is undesirable.
The laser beams produced by the R, G and B laser diodes can be or become misaligned for various reasons. For example, there will be some inherent misalignment that results from imperfect mechanical manufacturing of a projector system. Further, misalignment can occur due to mechanical modifications that occur to a projector system, e.g., if the projector system is dropped. Additionally, misalignment can also result from the thermal changes to the laser diodes, as well as aging of the laser diodes.